Preheat oven to 350°F (175°C).
Line a 9x13-inch metal baking pan with parchment paper and toast walnuts until fragrant.
In a large mixing bowl, whisk together melted butter and light brown sugar until smooth.
Beat in the eggs and vanilla extract until fully incorporated.
In a separate bowl, combine flour and salt.
Gently fold dry ingredients into wet ingredients, then fold in walnuts and any optional add-ins.
Spread the batter evenly in the prepared pan with an offset spatula.
Bake for 28–32 minutes, until edges are golden and center is set but fudgy.
Cool completely in the pan before slicing into 16 squares.